Method and apparatus for producing intense energetic gas discharges



Jan. 5, 1960 P. R. BELL ETAL 2,

METHOD AND APPARATUS FOR PRODUCING INTENSE ENERGETIC GAS DISCHARGES Filed July 24, 1958 3 Sheets-Sheet 1 GAS 35 SOURCE GAS SOURCE RF. VOLTAGE SOURCE i l i S INVENTORS Persa R. Bell 8 1 BY John S. Luce ATTORNEY Jan. 5, 1960 P. R. BELL ETAL 2,920,235

METHOD AND APPARATUS FOR PRODUCING INTENSE ENERGETIC GAS DISCHARGES Filed July 24, 1958 3 Sheets-Sheet 2 GAS 53 GAS 75 SOURCE SOURCE R. F. VOLTAGE SOURCE INVENTORS Persa R. Bell 8 BY John S. Luce ATTORNEY Jan. 5, 1960 P. R. BELL ETAL 2,920,235

1111mm AND APPARATUS FORPRODUCING INTENSE ENERGETIC GAS DISCHARGES Filed July 24. 1958 3 Sheets-Sheet 3 GAS -97 GAS 98 SOURCE SOURCE gi J g 87 f1 99 It 1o /113 ,111 \HT 2 4 R.F. VOLTAGE FD 112 SOURCE 114 116 9 121 A22 11 a A INVENTORS Persa R. Bell a BY John S. Luce ATTORNEY NIETHOD AND APPARATUS FOR PRODUCING INTENSE ENERGETKC GAS DISCHARGES Persa R. Bell and John S. Luce, Oak Ridge, Tenn., as-

signors to the United States of America as represented by the United States Atomic Energy Commission Application July 24, 1958, Serial No. 750,834

4 Claims. (Cl. 315-111) This invention relates to devices for producing energetic deuterium or tritium arc discharges.

Energetic arcs have been developed for use as a dissociating mechanism in thermonuclear devices. An example of the efficiency of a high current or energetic are as a dissociating mechanism is set forth in the applications of John S. Luce, Serial No. 728,754, filed April 15, 1958, entitled Method and Apparatus for Trapping Ions in a Magnetic Field, and Serial No. 738,242, filed May 27, 1958, entitled Device and Method for Producing a High Intensity Arc Discharge, now Patent No. 2,895,053, issued July 14, 1959. The latter application relates to an energetic carbon arc, and the former application relates to the use of the energetic carbon arc in a thermonuclear device. In one such thermonuclear device, high-energy molecular ions, such as D for example, are injected into a confining magnetic field perpendicular to the lines of magnetic force. At some point in the orbit of these ions in the magnetic field, a portion of them are caused to be dissociated and/or ionized by the carbon arc to form atomic ions. These resultant atomic ions have one-half the momentum of the original molecular ions that have an atomic weight of two and hencehave onehalf the radius of curvature in the field. The radius of curvature of the atomic ions depends upon the atomic weight of the injected molecular ions. If the center of the orbits of these atomic ions coincides with the axis of the magnetic field, the ions will circulate in a ring. If the center of the orbits and the axis of the machine do not coincide, the atomic ion orbit will precess about the point of origin of the atomic ion. The ions will circulate until a charge exchange occurs with one of the neutral gas atoms in the system, or until some other process causes the ions to be lost.

The energetic carbon arc produces low pressures under normal operation which, in part, accounts for its being very energetic. Despite the many desirable characteristics of the carbon arc, basic disadvantages exist. Energetic ions in the carbon arc collide with carbonions and lose much of their energy by Bremsstrahlung, scattering, and ionization. The amount of energy lost in such processes increases with the atomic number of the ions, so that for carbon ions the losses may be substantial, compared with deuterium, for example. It is very important in the process of building up a thermonuclear plasma that any losses of the energy imparted to the ions be held to a minimum.

Despite the disadvantages mentioned, the carbon arc has been utilized heretofore because conventional gas arc discharges cannot be operated in high magnetic fields at pressures low enough for them to become eflfective dissociating mechanisms. At operating pressures, such gas arcs cannot cause any trapping of atomic ions due to the large number of charge exchanges and to other processes that may cause loss of ions to occur.

Another problem associated with gas arcs is the intense heating of the electrodes with a consequent influx of impurities into the are.

A characteristic of energetic gas arcs is that some ofthe ions that are accelerated toward the cathode escape and pass around the outer surface of the cathode assembly and strike the supporting structure for the cathode and heretofore have served no useful purpose.

With a knowledge of the problems associated with creating a thermonuclear plasma, the shortcomings of conventional gas arcs as dissociating agents, the electrode heating problem, and the disadvantage of the carbon discharge, applicants have as a primary object of this invention production of an energetic discharge or arc which is capable of dissociating molecular ions passed there through.

An important object of the invention is the production of a highly energetic arc which will not be characterized by large Bremsstrahlung, scattering, and ionization losses.

It is another object of this invention to provide a device for operating a deuterium or tritium arc discharge under low pressures, and with a minimum of electrode heating.

It is still another object of this invention to provide a use for the large quantity of energetic ions that are accelerated past the cathode.

These and other objects and advantages will be apparent from a consideration of the following detailed specification and the accompanying drawings wherein:

Fig. 1 shows a schematic cross-sectional view of one embodiment of a device for'producing avery energetic deuterium or tritium arc discharge,

Fig. 2 shows a schematic cross-sectional view of another embodiment of a device for producing an energetic deuterium or tritium arc discharge,

Fig. 3 shows a cross-sectional view of a use for the arc discharge of Fig. 2, in the propulsion of energetic ions, and

Fig. 4 shows a cross-sectional view on the line 44 of Fig. 3 showing the manner in which the cathode is sup ported. 7

The objects stated above have been achieved in the present invention by providing novel means for producing unique energetic arcs in deuterium or tritium which have a low atomic number, and therefore low Bremsstrahlung, scattering, and ionization losses. These unique arcs have also proved capable of dissociation of molecu tritium di s-' lar ions, unlike conventional deuterium or charges.

The apparatuses for producing the discharges described;

herein are similar to Fig. 1 of the application of John S. Luce, Serial No. 748,771, filed July 15, 1958. V In said application, the arc discharge is controlled so thatit terminates from Within a hollow elongated cathode and the rate of gas feed is maintained at a valuesuch that adequate space-charge neutralization is provided inside the cathode but not in the main volume. The arc dis.- charge depends upon emission across the magnetic field within the hollow cathode. V

In accordance with the present invention, in the embodiment illustrated in Fig. 1, the cathode electrode has a small hole bored through the center of the cathode and gas is fed to the face of the cathode Where space charge into the vacuum chamber and thus is essentially com- It is this anode design .and the axial potential gradient that exists in the arc Whichpermits pletely ionized.

the arc to be operated in low pressures and at voltages and currents that permit the arc to be energetic.

.1. 9,920,285 I I V, 1

The arcs described herein operate under substantially high voltage and current, and as a result, heating of the electrodes is a problem. This problem has been solved by using electrodes which are tungsten or any other high melting point conductive material and that is alsov a ,go od thermal conductor. Theelectrodes areinserted into thlck water-cooled copper jackets. The bore'of the jacketmay suitably be undersized to the extent that heating to several hundred degrees is required for insertion of, the electrodes. During operation of such a jacketed electrode, thedifierential expansion produces a very tight fit and therefore good'heat transfer. Carbon may be used for the anodes but is undesirable for the reason that it is a poor thermal conductor under high temperature and it is not possible to provide sufficient cooling of the anodes to prevent them. from partially vaporizing and emitting carbon particles into thearc volume. Such particles are undesirable. because of losses caused by Bremsstrahlung, scattering, andionization as discussed above. 7 7

Referring now. to Fig. 1', which illustrates one embodimentofan apparatus in which theprinciples of 'this'invention may be carried out, a centrally bored cathodeelectrode '1;is mounted in a copper block 3, and a hollow elongated cup-shaped anode electrode 2 is mounted in a copper block 4. Floating cathode shields 5 and 6' are mounted by-insulators 8 and 9,- respectively, to an outer chamber wall 13. Anode shield 7is mounted by an insulator 10 to the chamber wall 13. A strong magnetic field, for example, 6000 gausses, is provided by magnets 11 and 1-2, the direction of the field'indica'ted by the arrow H. Deuterium or tritium gas is fed from a source 14, through tube 15, and through the hole; 34 to the face of the cathode. The cathode hole is of sufficient size to permit space-charge neutralization at the face of the cathode. The cathode and'anode are cooled by a'cooling fluidwhichis passed through tubes33, mountedin blocks 3 and 4, from a source not shown. Deuterium or tritium gas is fed' from a source 35, through tube 36, and through tube'37 to, theinsideand' at the base of the anode 2.

An outer vacuum chamber 21 is formed by the walls; 13

and. is connected to a vacuum pump by the tube 23. An

inner'vacuum chamber 20 is formed by the walls 17, as

shown, and is connected to a vacuum pump by thetubular member 22. 'Bafiles 18 are provided adjacent to the cathodel, and baffles 19 are provided adjacent to the anode 2., These bafllcs areinsulatingly'supported in the; An arc initiating assisting means inside. chamber walls. such as an R.F. voltage source 31, which may-be a conventional? welding source, is connected at one end to anode 2*by a lead 32, and at its other'end to cathode 1 by lead 30, switch 29, and lead 28. An operating potential is connectedito' the anode and cathode by a variable D.C. source, such as a variable-tap multi-cell battery'25. Batter-y 25 is connected at one side tocathode 1 by alead 24, and is connected atits other side to anode 2 by a switch 26, and lead; 27."

The blocks 3 and 4 are insulatingly supported on the chamber walll3 by means not shown. In one example ofthe apparatus, illustrated in Fig. l, the cathode was about 5 inches long, had an outer diameter of 0.5 inch, and a gas-feed bore of 0.125 inch. The anode was -5 inches long, had an external diameter of 0.75 inch, and

an internal diameter of 0.625 inch." With these dimen sions, .there is a clearancebetween the arc 16' and the inside surface of the hollow anode of approximately- Msinch. a

In one operation of the'apparatus of Figfl at startup, gas is fed fromr source 14, through tube and bore 35 of cathode 1 to the face of said cathode and gas is fed from source '35, and through tube 36 to the inside of anode 2 at the base thereof until the pressure in the anode and at the face of the cathode and in thechamber 20 reaches a value of 3X10 mm. Hg, and then an R.F.

voltage source 31 is applied across the electrodes 1, 2, and a .D.C. voltage source is Connected across the electrodes; After-the are 16' is initiated; the R.F. source 31 is disconnected by the switch 29. The pressure in chamber 20 is then gradually reduced until it reaches a value of approximately 5 19- mm. Hg, or lower. The pressure in'outer chamber 21 is maintained at a value of approximately 3 X 10 mm. Hg, or lower. The are may be initiated at startup with a pressure lower than 3 l0- mm. Hg provided the R.F. voltage 31 and the operating potential are correspondingly increased.

There are several other conventional methods for helping to initiate an arc discharge between the electrodes 1.: and 2 other than that specified above, such as applying a very high starting potential between the electrodes, heating the electrodes until they are completely out-gassed and then applying a high starting potential between the electrodes, or by providing an auxiliary electrode adjacent to the cathode and momentarily touching the cathode with said auxiliary electrode while atthe same time applying a voltage between-the-cathode and auxiliary electrode until an arc is struck between them and then. gradually separatingsaid electrodes while applying an are potential auxiliary electrode.

between the cathode and anode, and then removing the.

During all stages of operation of a, device having the; dimensionsrreferred to above, a magnetic field strength.

of a selected value in the range'of 3000 to 6000 gausses is maintained by the magnets 11 and 12 as the pressure in chamber 20 is lowered, the voltage across the elec trodes is increased and when the pressure in chamber 20:

reaches, a normal operating value, forexample 5x10 mm. Hg,. the voltage .across the electrodes 1 and 2 is. approximately- 200, volts and the arc current reaches a: value of approximately 15.0 amperes for a four inch arc...

gas within the anode. Also it is necessary that the bores, oi'thecathode,andanode are in accurate alignment with.

the magneticv field;

Uhd r. normaloperating conditions, electrons are accelerated toward the anodeand ions are accelerated along.

the maguetiefield toward the cathode by an axial potentialgradientin the'arc. .A.large-number of the ions that areiacce'lerated, tothe cathode and that recombine willthen be prevented. from flowing back as neutral particles. by the bafiles 18. between the cathode and the working.

volume ofvthearc. Bafiies 18. and 19 also serve .as a means for preventing any gas particles that escapefrom the'electrodes' from entering into the working volumeof the arc without'first becoming ionized by the discharge.

The are described above is an efiicient dissociating device. It hasbeen determinedthat this are has break-upv efiiciencies of 15% when used for dissociating 20 kev..

deuterium molecularions, a

Referring now to Fig. 2, whichillustrates another embodiment of an apparatusin which the principles of this invention may be; carried out, there is shown an example ofja'device in whichan energetic deuterium or tritium gas are; discharge can .be initiated and sustained between a hollow, cup-shaped cathode and a hollow cup-shaped anode; The device set forthin-Fig. 2 is similar to Fig. 1 of theyapplication of John S. Luce, Serial No. 748,771, filed July '15, 1958,"a'forementioned, except for the. fol: lowing differences. Magnetic mirrorcoils are provided in lieu of end magnets. Also, gas is fed to the inside base of the follow anode in Fig. 2 of the, instant application. t V g V j In'Fig; 2, an elongated hollow cup-shapedcathodeelectrode-40"is-mou nted in a copper hub 42 which is insulat-t ingly supported bymember 78 to the o'uter chamber wall 52; An elongated hollow cupshaped anode electrode 41 is mounted. in acopper .block 43 which is insulatingly ing cathode shields 44 andf45 are mounted by insulators 47 and 48, respectively, to the outer chamber wall 52. Floating anode shield 46 is mounted by an insulator 49 to the chamber wall 52. A strong magnetic mirror field, for example, 6000 gausses, is provided by the magnetic mirror coils 50 and 51, the direction of the field indicated by the arrow H. Deuterium or tritium gas is fed from source 53, through tube 54, and through tube 74 to the inside of the hollow cathode 49 in an area adjacent to the base thereof. Deuterium or tritium gas is fed from a source 75, through tube 76, and through tube 77 to the inside of the hollow anode 41 in an area adjacent to the base thereof. The cathode 40 and anode 41 are cooled by acooling fluid which is passed through tubes 73, mounted in the members 42 and 43, respectively, from a source not shown.

An outer vacuum chamber 60 is formed by the walls 52, and is connected to a vacuum pump by the tube 62. An inner vacuum chamber 59 is formed by the walls 56, as shown, and is connected to a vacuum pump by the tubular member 61. Apertured bafiies 57 are provided adjacent to the cathode 4i), and apeitured bafilles 58 are provided adjacent to the anode 41. These bafiies are insulatingly supported and are designed to closely fit the arc discharge the same as the hollow anode. An arc initiating assisting means such as an RF. voltage source 71, which may be a conventional welding source, is connected at one end to anode 41 by a lead 72, and at its other end to cathode 40 by a lead 70, switch 69, and lead 68. An operating potential 64 is connected between the anode and cathode. Potential 64, which may be a variable-tap, multi-cell battery is connected at one side to cathode 40, by a lead 63. and at its other side to anode 41, by a lead 65, switch 66, and lead 67.

In one example of the device, illustrated in Fig. 2, the cathode was 5 inches long, had an outer diameter of 0.75 inch, and an inner diameter of 0.375 inch. The anode was 5 inches long, had an outer diameter of 0.75 inch, and an inner diameter of 0.5 inch. As in Fig. 1 above, there is clearance between the arc discharge 55 and the inner surface of hollow anode 41 of approximately ,4 inch.

In accordance with the invention, as illustrated in Pig. 2, by regulating the rate of gas flow to the hollow cathode, it is possible under proper pressure conditions to provide an arc discharge which runs from the inner surface of the cathode. Under these conditions, substantially complete space-charge neutralization is provided inside the cathode. Under low pressures, for example 3x10 mm. Hg. or lower, the arc in the modification of Fig. 2 is a hollow cylinder and the arc current is carried in this cylinder close to the inside cathode wall. The are depends upon emission across the magnetic field within the hollow cathode, no'decrease in emission occurs when the arc is operated in a strong magnetic field, for example, a field of 6000 gausses.

There are at least three unique characteristics of th deuterium or tritium arc discharge, illustrated in the apparatus of Fig. 2, which distinguishes it from other arcs and which permits it to be very energetic. The first characteristic is the cross field emission, achieved by provision of a large surface area from which electrons may enter the system. The deuterium or tritium gas which is fed into the inside of the hollow cathode is substantially completely ionized before it leaves the cathode. Since there is adequate space-charge neutralization inside the cathode, it is possible to decrease the pressure without affecting the emission characteristics of the arc. The second distinguishing characteristic of the arc of Fig. 2 is the existence of an axial potential gradient in the arc. Electrons are accelerated radially from the inside wall of the cathode. Some of those electrons which suifer collisions before returning to the wall are trapped close to the cathode inside wall and thereupon are drawn out of the cathode'by this'axial'potential gradient. The third distinguishing characteristic of the arc of Fig. 2 is the fact that it fits very close to the inside wall of the hollow anode and gas that is fed into the base of said anode is substantially completely ionized before it leaves the anode.

In one operation of the device of Fig. 2, having the dimensions referred to above, at startup, gas is fed from source 53 and through tubes 54 and 74 to the inside of cathode 40, and gas is fed from source 75, and through tubes 76 and 77 to the inside of anode 41 until the pressure in the cathode 40, in the anode 41, and in chamber 59 reaches a value of approximately 3x10- mm. Hg, and a RF. voltage source 71 is then applied across the electrodes 40, 41. Also, a DC voltage source 64 is connected between the electrodes. The are is initiated at the faces 80 and 81 of the cathode 40 and anode 41, respectively. After the arc 55 is initiated, the R.F. source 71 is disconnected and the gas feed from sources 53 and 75 is reduced until the arc moves from the faces 80 and 81 of the electrodes into the interior of the electrodes which may be observed visually. The gas feed to the arc is maintained at a rate just sufiicient to provide complete space charge neutralization within the hollow cathode 40 and to provide substantially complete ionization of the gas within hollow anode 41, while the pressure in cham-- ber 59 is gradually reduced until it reaches a value of approximately 3X10" mm. Hg or lower. The pressure in outer chamber 60 is maintained at a value of approxi-' mately 3 X 10* mm. Hg or lower.

During all stages of operation of the device of Fig. 2 having the dimensions above, a magnetic field strength of about 6000 gausses is maintained by the magnetic mirror coils 50 and 51. As the pressure in chamber'59 is lowered, the voltage across the electrodes is increased and when the pressure in chamber 59 reaches a normal operating value, for example 3x10 mm, Hg, the voltage across the electrodes 40 and 41 is approximately 400 volts and the arc current reaches a value of approximately amperes.

As in Fig. 1 above, there are several other conventional ways, as discussed above, for striking the arc andat lower pressures.

In order for the arc of Fig. 2 to operate from within the cathode and under the preceding conditions, it is necessary that the gas flow be maintained at a rate sufficiently low that adequate space charge neutralization is provided inside the cathode, and that the bore of the cathode is in accurate alignment with the magnetic field.

The baflies 57 and58 of Fig. 2, referred to above, serve the same purpose as the bafiies 18 and 19 of Fig. 1.

The are described for Fig. 2 above is an eflicient dissociating device. It has been determined that this arc has break-up efficiencies of 25% when used for dissociating 20 kev. deuterium molecular ions.

7 The arc formed by the device of Fig. 2, as discussed above, is substantially a hollow arc. .In addition to its usefulness for dissociating and/or ionizing of molecular ions, this arc is useful in reducing the instreaming of cold neutrals from the vessel Walls into the plasma of a thermonuclear machine such as disclosed in the application of John S. Luce, Serial No. 728,754, aforementioned. In using the deuterium or tritium arc of Fig. 2 in the machine therein described, the magnetic mirror coils at the ends of said machine permit the hollow arc to be enlarged in the center portion where the molecular ions are injected. Under such conditions, the thermonuclear plasma is confined within the hollow arc and some of the energetic particles from the plasma will penetrate the arc and strike the walls of the machine. However, low energy neutrals resulting from this bombardment and other processes cannot pass back through the arc barrier and enter the plasma without being ionized in the arc. The ions thus formed are then collimated by the magnetic field and accelerated out of the are by the potential gradients that exist inth'e arc discharge. It has been determined that a substantial quantity 7 of the energetic ions that; are accelerated toward the cathode of; Fig. 2 -by the large potential gradients: in the are spread out on their way to the cathode and strike the end baffle 57; This propulsion of energetic ions may be useful in the ion propulsion of space vehicles in regions of negligible atmospheric pressure. Fig. 3 shows a use for the discharge of Fig. 3 for propulsion of energetic ions. The batfies, the end cathode shield, the hub around the cathode, and the end wall of the device ofFig. 2 have been removed, as illustrated in the device of Fig. 3. Also, the cathode is supported by a plurality of-radially-extending spokes, and no vacuum pump is connected to the chamber since the device, when used in a space vehicle, will be connected directly to the negligible atmospheric pressure in space through the open end wall of' the chamber. With the provision of spokes for mounting the cathode and the elimination of thebaflies, end shield, and'end-wall of the chamber, the energetic ions that spread out on their accelerated return toward the cathode will be propelled past the outside of the cathodeand through the spokes into space.

Referring now to Fig. 3, which illustrates an apparatus for propulsionof energetic ions, an elongated cup-shaped cathode electrode 85 is mounted to a plurality of radially extending spokes 87, 88 and 89 which are in turn insulatingly supported to'the chamber Wall 102. An elongated hollow cup-shaped anode electrode 86 is mounted ina copper block 90-which is insulatingly supported by a member 109 to the chamber wall 102; The anode 86 is cooled by a plurality of cooling tubes 110, mounted in block'90, by passing a cooling fluid through these tubes from a cooling source, not shown. Gas is fed from a source 97-, through :tube 100, and through tube 96 to the interior of hollow cathode 85 at the base thereof. Gas is fed from; a source 99, through tube 101, and through tube 99 to the interior of hollow anode 86 An annular cathode shield 91 is mounted to the chamber wall 102 by insulators 94 and 95. Annular magnetic mirror coils 103 and 104 are provided. Mirrorcoil 103 is mounted to the chamber wall 102 by insulators 105 and 106. Mirror coil 104 is mounted to the chamber wall 102 by insulators 107 and 103. An anode shield 92 is mounted to chamber wall 102 by an insulator 93.

A source of'R.F. voltage 111 is connectedat one end to cathode 85 by leads 112 and 113, and at its other end to anode 86 by lead 114, switch 115, lead 116, and lead 117. A source of operating potential 118, such as a variable-tap multi-cell battery, is connected at one side to cathode 85 by a lead 119 and lead 113, and at its other side to anode 86 by lead 120, switch 121, lead 122, and lead 117.

- The are discharge 123 of Fig. 3'may be initiated in any of the conventional ways as discussed above. As in Fig.2, thedischarge of Fig. 3 is terminated from within the hollow electrodes and said electrodes closely fit the discharge. so that the feed gas fed to the interiors ofboth electrodes is substantially completely ionized before it leaves the electrodes. The are discharge 125 is made up of substantially completely ionized ions, and electrons; These energetic ions are accelerated'toward the cathode by the potential gradients in the are and are collimated by the streams 124 of ions.v As indicated above, these streams of energetic ions may be propelled into outer space when the device is used in a space vehicle;

' Fig. 4. shows a sectional view of the cathode supporting structure when viewed from the sectional line 4-4 of Fig. 3. As shown on the drawing the radial spokes 237,

8.8 and 83 are shown mounted to the cathode 85 and to the chamber. wall 102.

Thearcs described above have been operated with other gases, for example, air, hydrogen, argon, nitrogen and helium, and. may. be; operated with most other gases.

However, the. use of deuterium or tritium is preferred.

for thermonuclear reactions since they introduce the least amount of extraneous material intothe discharges.

The arcs described above can also be used to provide a source of energetic ions and electrons.

This invention has been described by way of illustrationrather than limitation, and it shouldbe apparent that the invention is equally applicable in fields other than those described.

What is claimed is:

1. A device for establishing a high-intensity or energetic gas are discharge which comprises a containing vessel, means for evacuating said vessel to a selected pressure, an elongated cathode electrode provided with a central axial chamber, and an elongated hollow cup-shaped anode electrode having a larger inner diameter than that of said c'athode,"said anode and cathode having a common axis and being widely spaced apart in confronting relation Within said vessel; a plurality of annular baflles disposed adjacent to the confronting faces of said anode and cathode and having an inner diameter substantially equal to that of said anode; means for establishing a magnetic field within said chamber oriented parallel to said axis, means for feeding gas at controlled rates to the interiors of both said electrodes so that space-charge neutralization occurs at said cathode, means for striking and maintaining an energetic arc discharge between said electrodes, and means for cooling said electrodes, said discharge forming a cylinder extending into said anode chamber so that gas fed to said anode is completely ionized therewithin.

2. The device as set forth in claim 1 in which the cathode chamber is hollow and cup-shaped, and the feed gas rate is controlled so that the are discharge terminates from within said hollow cathode and substantially complete space-charge neutralization occurs within said hollow cathode.

3. The device as set forth in claim 1 in which the cathode chamber is a small axial aperture, and thefeed gas rate is controlled so that the substantially complete space-charge neutralization occurs within said aperture and at the face of said cathode.

4. A device for producing a directed stream of energet'ic' ions for ionic propulsion comprising a vessel having an open end, an elongated hollow cup-shaped cathode electrode and an'elongated hollow cup-shape anode electrode having a common axis and being widely spaced apart in confronting. relation within said vessel, said anode havinga larger inner diameter than that of said cathode, means for establishing a magnetic field within said vessel oriented parallel to said axis, means for feeding gas at controlled rates to the interiors of both said .electrodes, means for striking and maintaining an energetic arc discharge of ions and electrons between the inner surfaces of said electrodes, said arcsubstantially References Cited in the file of this patent UNITED, STATES PATENTS 2,009,555 Mathiesen July 30, 1935 2,294,498 Heindlhofer Sept. 1, 1942 2,728,877 ,Fischer Dec. 27, 1955' 2,819,427 Noskowicz Jan. 7, 1958 2,826,708 Foster Mar. 11,, 1958- Von Ardenne Q,--.---.-.-...- Mar. '11,; 195.8

UNITED STATES PATENT OFFICE CER F ICATE or CORRECTION Patent No, 2,920.235 I January 5, 1960 I I Persa H. Bell et a1. I

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said'Letters Patent should read as corrected below.

' Column 1 'lines 25 and 26 for "Patent No. 2,895,053

issued July 14, 1959" read Patent No. 2,920,234, issued January 5, 1960 Signed and sealed this 26th day'of July 1960.,

(SEAL) Atiest:

KARL H. AXLINE ROBERT w soN Attesting Officer I Commissioner of Patents UNITED STATES PATENT- OFFICE v CERTIFICATE OF CORRECTION a Patent No. 2,920,235 d January 5, 1960 v Persa R. Bell et a1.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said-Letters Patent should read as corrected below.

' Column 1 lines. 25 and 26 for "Patent No. 2,895,053 issued July 14, 1959-" read Patent No. 2,920,234 issued Januaryfi, 1960 v Signed and sealed this 26th day'of July 1960.

(SEAL) Attest:

KARL H. AXL IN ROBERT c. WATSON Attesting Officer Comnissioner of Patents 

